1. Field of the Invention
This invention relates to electronic musical tone synthesis and in particular is concerned with the generation of polyphonic tones in response to keyswitches actuated in an array of keyswitches.
2. Description of the Prior Art
Electronic keyboard musical instruments can be classified as either being monophonic or polyphonic tone generators. An instrument of the monophonic type is limited to generating only a single tone at one time. If more than one keyswitch is actuated simultaneously, all but one of the keyswitches is ignored so that only a single tone is generated. A polyphonic tone generator is an instrument capable of generating a plurality of tones in response to a corresponding actuation of keyswitches.
Digital tone generators that operate by having a stored waveshape consisting of a set of data points defining a single period of a musical waveshape can be implemented as a polyphonic musical instrument by having an independent memory means associated with each of a plurality of tone generators. These tone generators are assigned to actuated keyswitches. An instrument of this type is described in U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer."
A single waveshape memory can also be shared between several tone generators. A system for utilizing a single waveform memory in a time shared, or multiplexing, fashion is described in U.S. Pat. No. 3,743,755 entitled "Method And Apparatus For Addressing A Memory At Selectively Controlled Rates." A calculator is used for continuously computing a set of numbers, each of which defines a different spacing between the addresses of the waveshape memory. The numbers calculated are each periodically increased by their own value and the individual results identify addresses for the waveshape memory. The numbers are used in turn to address the memory to obtain waveshape sample points corresponding to each of the several tone generators.